This invention relates to plasticating using a screw rotatable within a barrel to extrude or inject molten resinous material. More particularly, this invention relates to a longitudinal portion of the screw designed to recirculate material for thorough mixing and melting.
A plasticating apparatus commonly used today receives polymer or thermoplastic resin pellets, granules or powders, from an inlet port, then heats and works the resin to convert it into a melted or molten state. The melt or molten material is delivered under pressure through a restricted outlet or discharge port to make the finished article. It is desirable that the molten material leaving the apparatus be completely melted and homogeneously mixed, resulting in uniform temperature, viscosity, color and composition.
The basic plasticating apparatus includes an elongated cylindrical barrel which usually is heated at various locations along its length. An axially supported and rotating screw extends longitudinally through the barrel. The screw is responsible for forwarding, melting, pressurizing and homogenizing the material as it passes from the inlet port to the outlet port. The screw has a core with a helical flight thereon and the flight cooperates with the cylindrical inner surface of the barrel to define a helical valley for forward passage of the resin to the outlet port.
Since there are several different types of thermoplastic resins or polymers, and with each having different physical properties and characteristics, there are different screw configurations. In general, however, the typical plasticating screw has a plurality of sections along its extended axis with each section being designed for a particular function. Ordinarily, there is a feed section, a transition section and a metering section in series. In the art, the transition section has been referred to interchangeably as the intermediate, compression or melt section.
The feed section extends forward from the inlet port of feed opening where solid thermoplastic resins, in pellet, granular or powder form, are introduced into the apparatus and pushed forward by the screw along the inside of the barrel. The resin is then worked and heated in the transition section so that melting occurs. After approximately 40 to 80 percent of the resin has been melted, solid bed breakup occurs, and solids become randomly dispersed within the melt. It is important to note that most melting initially occurring in the transition section takes place at or near the heat source of the barrel. Then, melting and mixing become enhanced as solids subsequently become broken up, redistributed and dispersed within the melt. To assure a homogeneous melt, therefore, it is important that the transition section enhances turbulent flow, as opposed to laminar flow, so that all the resin comes within the heating vicinity of the barrel or is dispersed within the melt. Otherwise, the presence of minute unmelted resin particles will appear in the finished article.
Typically the transition section has a decreased root depth of the helical valley, as compared with the feed section, to reflect the volume reduction due to melting of the feed and the elimination of air spaces between the solid particles. The transition section leads to the metering section. The metering section, as one of its intended functions, provides a constant flow of molten material toward the outlet port. In addition, it is important that the metering section melt any unmelted solids and mix and maintain the molten resin in a homogeneous and uniform composite until discharged through the outlet port.
The melting and mixing functions are enhanced by using screw configurations which increase the compression and shearing force applied to the resin by the screw. Compression and shearing increase turbulence, tumbling and mixing of material, resulting in increased homogeneity. The process also converts mechanical energy to thermal energy, resulting in a temperature rise of the material. While higher shear rates provide better mixing, the higher temperature resulting therefrom can cause excessive degradation of the resin or cause processing problems downstream of the screw. A wide variety of plasticating screws of different designs have been developed, therefore, to address this problem.
U.S. Pat. No. 4,639,143 discloses an improvement in plasticating screws with a portion in at least one of the sections of a typical apparatus having a plurality of discrete grooves arranged in a noncontinuous helix cut into the root of the screw, within and preferably parallel to the helical valley. The grooves cooperate with the helical flight in advancing the molten resin forward along the longitudinal axis of the apparatus towards an outlet port and are dimensioned to receive the resin in a manner whereby appropriate shear force is applied where needed. The resin passes down the helical path flowing from groove to groove in random fashion, which enhances mixing. Molten resin easily flows forward through the advancing grooves, where it is subjected to lower shear forces, while unmelted solids flow over grooves and barrier lands with proportionately higher shear forces than in the grooves alone. As a result, these higher shear forces are intended to break up the residual solid materials, increase temperature to help with the melting process and enhance mixing to allow for the transfer of heat by conduction.
Although this configuration may satisfy many general needs, thermal and composite mixing can be improved even more for various thermoplastic resin and polymer materials by having a novel longitudinally cylindrical or tapered portion that allows for a continuous back flow and recycling of molten material. The objective of the present invention is to substantially improve on prior screw designs by providing a plasticating screw having a modified longitudinal melting and chaotic mixing portion. At the same time this invention permits greater temperature control to avoid the overheating or degradation of the resin. Ultimately, the primary objective of the instant invention is to homogeneously mix select resins, resulting in a completely molten material having uniform temperature, viscosity, color and composition.
The present invention provides an improvement in a plasticating apparatus having a heated barrel with an axial length wherein solid material is introduced through an inlet port and exits as molten material through an outlet port of the barrel. The heated barrel has an inner-wall and a screw rotatably supported therein. The screw further comprises at least one helical flight extending along its length to define a helical channel with the inner-wall.
Said screw typically comprises at least a feed section cooperating with said inlet port, an intermediate melt section, and a metering section cooperating with said outlet port. A longitudinal portion is included in at least one of said sections of the screw, having a plurality of noncontinuous advancing grooves with a closed-ended arranged in said helical channel. The advancing grooves are dimensioned to receive said material therein as the material is being conveyed toward the outlet port. The improvement herein comprises a plurality of noncontinuous retracting grooves traversing the axial length of the longitudinal portion of the screw in a reverse direction as compared with the helical direction of said flight. Each retracting groove passes through the helical flight at least once so that molten resin can back flow, recirculate and chaotically mix within the longitudinal portion of the instant invention.
In accordance therewith, the melting and mixing functions of the screw are enhanced, while better controlling the temperature within the plasticating apparatus. The instant invention further reduces resin degradation and increases uniformity of the temperature, viscosity, color and composition of the molten material ultimately discharged downstream. Many other objectives and features of the present invention will be obvious to those of skill in the art upon contemplation of the entire disclosure herein in connection with the accompanying drawings.